When certain polarizable solid particles are dispersed in an electrically non-conducting hydrophobic liquid, the resulting suspensions exhibit peculiar rheological properties under the influence of an electric field. These systems show a dramatic increase in viscosity and modulus with applied voltage, in some cases literally being transformed from a liquid to a virtual solid upon the application of the electric field. This change is reversible and typically takes place in a matter of milliseconds. Materials which exhibit this phenomenon are called electrorheological (ER) or eletroviscous (EV) fluids, and have been known for at least the last fifty years. These fluids find utility in such areas as torque transfer and mechanical damping applications.
The early ER fluids comprised such systems as starch dispersed in transformer oil or silical gel dispersed in kerosine or mineral oil. Since these early discoveries, only a relatively small number of new systems, and improvements over old ones, have emerged in this art.
Thus, for example, Goossens et al., in U.S. Pat. Nos. 4,702,855 and 4,645,614, disclose ER fluids based on aluminum silicates in an electrically non-conducting liquid and silica gel in silicone oil, respectively. In each of these patents, the contribution to the art was an improved electroreactivity, as well as improved stability, over a wide temperature range. This end was accomplished by the addition of certain functional polysiloxane dispersants to the ER fluid formulations.
Unfortunately, the compositions of Goossens et al., cited supra, seem to rely on the adsorption of water onto the respective dispersed particles for their ER effect. Thus, the water content limits the upper use temperature of such fluids as it desorbs upon heating. This drawback was addressed by Filisko et al. in U.S. Pat. No. 4,744,914, which teaches an ER composition based on a particulate crystalline zeolite. The zeolite does not readily release water and allows the resulting ER fluids to be used at a typical temperature of about 120.degree. C.
In addition to the above mentioned disadvantages, the presence of water in the dispersed phase of an ER fluid is generally undesirable since it can promote corrosion of metal parts as well as lead to excessive leakage current during operation, the latter being directly related to the power consumption of a given device. Leakage current is defined herein as the electrical current passing through the ER fluid when a given voltage is impressed across the fluid volume. The leakage current is often expressed as a current density in order to normalize this measurement with respect to device geometry.
Bloch et al., in U.S. Pat. No. 4,687,589, disclose ER systems wherein at least the dispersed phase is substantially anhydrous. These systems, which are based on organic conductors and semiconductors, are capable of operating as ER fluids and are said to result in reduced current densities relative to prior art fluids.
Other polymeric dispersed phases which are suitable for use in ER fluid compositions are described by Stangroom in Great Britain patent specification No. 1,570,234 (to the Secretary of State for Defense, London). This disclosure teaches electroviscous fluid compositions comprising various watercontaining polymers having free or neutralized acid groups dispersed in an electrically non-conducting oleaginous vehicle. The polymer must have a density of less than 1.8 g/cm.sup.3 and have a specific degree of water absorbency in order to be suitable for the intended applications. Although these systems are stated to provide an enhanced electroviscous effect, they still suffer from the inclusion of a significant amount of water. Furthermore, a variety of acrylic-type polymers illustrate the invention but no silicone material is mentioned in GB No. 1,570,234 as a possible candidate for use as either the polymer or the oleaginous phase.
As can be seen from the above examples, most of the disclosures in the ER fluid art focus mainly on the dispersed phase. This seems to be the case since the constitution of the continuous phase is generally not critical as long as this fluid phase is an electrically non-conducting hydrophobic liquid. Thus, even though various silicone fluids, such as polydimethylsiloxane and fluorosilicone oil, have been employed in the art as the continuous phase, an all-silicone ER fluid has not as yet been disclosed, according to applicant's best knowledge.